Method and apparatus for transmitting frame in wireless lan

ABSTRACT

Disclosed are a method and an apparatus for transmitting a frame in a wireless LAN. The method for transmitting a frame in a wireless LAN may comprise the steps of: transmitting, by an AP, an RTS frame for medium protection to a first STA set; receiving, by the AP, CTS frames, in sequence, from each of a plurality of STAs included in a second STA set in response to the RTS frame, the second STA set being included in the first STA set; and transmitting, by the AP, each of a plurality of PPDUs to each of the plurality of STAs via each of a plurality of subbands for each of the plurality of STAs on an overlapping time resource.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to wireless communication and, mostparticularly, to a method and apparatus for transmitting a frame in awireless LAN.

Related Art

The range of channel bandwidths that are available for usage in thelegacy wireless LAN system has become diverse from 20 MHz to 160 MHz.Accordingly, deciding an adequate channel bandwidth for thecommunication between a transmitting user equipment and a receiving userequipment has become a crucial factor in determining Wi-Fi performance.

In order to decide an adequate channel bandwidth for the communicationbetween the transmitting user equipment and the receiving userequipment, a dynamic channel bandwidth configuration protocol, which isbased on a RTS(request to send) frame and a CTS(clear to send) frame,has been developed starting from IEEE 802.11ac. The initial RTS frameand CTS frame have been devised to resolve the hidden node issue and toreduce data frame collision overhead. Before transmitting a data frame,the transmitting user equipment transmits a RTS frame to the receivinguser equipment. After receiving the RTS frame, the target user equipmentsends a response to the transmitting user equipment by using the CTSframe. Third party user equipments that have received the RTS frame andthe CTS control frame may delay medium access for a predetermined periodof time for the protection of data frame that are to be transmittedlater on.

Referring to the dynamic channel bandwidth configuration protocol, whichhas been supported starting from IEEE 802.11ac, the transmitting userequipment may transmit a RTS frame through a wide band exceeding achannel bandwidth of 20 MHz, and the target user equipment may send aresponse with a CTS frame in accordance with a channel bandwidth thatcan be currently used by the corresponding target user equipment. Forexample, in case the transmitting user equipment wishes to use a channelbandwidth of 160 MHz, the RTS frame is transmitted through a 160 MHzchannel bandwidth. In case the channel bandwidth that is currentlyavailable for usage of the target user equipment corresponds to 80 MHz,the target user equipment transmits the CTS frame through a 80 MHzchannel bandwidth. In case the transmitting user equipment that hastransmitted the RTS frame receives a CTS frame through a channelbandwidth of 80 MHz, the data frame that is transmitted afterwards tothe target user equipment by the transmitting user equipment should beequal to or smaller than the 80 MHz channel bandwidth.

SUMMARY OF THE INVENTION Technical Objects

An object of the present invention is to provide a method fortransmitting a frame in a wireless LAN.

Another object of the present invention is to provide an apparatusperforming a method for transmitting a frame in a wireless LAN.

Technical Solutions

In order to achieve the above-described technical object of the presentinvention, according to an aspect of the present invention, a method fortransmitting a frame in a wireless LAN may include the steps oftransmitting, by an AP(access point), a RTS(request to send) frame formedium protection to a first STA(station) group, sequentially receiving,by the AP, CTS(clear to send) frames from each of multiple STAs beingincluded in a second STA set as a response to the RTS frame, wherein thesecond STA set is included in the first STA set, and respectivelytransmitting, by the AP, each of multiple PPDUs (physical layer protocoldata unit) to each of the multiple STAs through each of multiplesubbands for each of the multiple STAs within an overlapping timeresource.

In order to achieve the above-described technical object of the presentinvention, according to another aspect of the present invention, anAP(access point) transmitting a frame in a wireless LAN may include aRF(radio frequency) unit being configured to transmit or receive radiosignals, and a processor being operatively connected to the RF unit,wherein the processor may be configured to transmit a RTS(request tosend) frame for medium protection to a first STA(station) group, tosequentially receive CTS(clear to send) frames from each of multipleSTAs being included in a second STA set as a response to the RTS frame,and to respectively transmit each of multiple PPDUs (physical layerprotocol data unit) to each of the multiple STAs through each ofmultiple subbands for each of the multiple STAs within an overlappingtime resource, and wherein the second STA set may be included in thefirst STA set.

Effects of the Invention

By transmitting a plurality of frames from multiple STAs over anoverlapping time resource, the communication efficiency may be enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a concept diagram illustrating the structure of awireless local area network (WLAN).

FIG. 2 illustrates a concept diagram showing a method for using a RTSframe and a CTS frame for resolving a hidden node issue and an exposednode issue.

FIG. 3 illustrates a concept diagram showing a CTS-to-Self Mechanism.

FIG. 4 illustrates a concept diagram showing a medium protection methodwhen performing OFDMA communication based communication in a wirelessLAN according to an exemplary embodiment of the present invention.

FIG. 5 illustrates a concept diagram of a RTS frame according to anexemplary embodiment of the present invention.

FIG. 6 illustrates a concept diagram showing a collision between a BSSsupporting DL MU OFDMA transmission and a BSS not supporting DL MU OFDMAtransmission.

FIG. 7 illustrates a concept diagram showing a collision between a BSSsupporting DL MU OFDMA transmission and a BSS not supporting DL MU OFDMAtransmission.

FIG. 8 illustrates a medium protection method according to an exemplaryembodiment of the present invention.

FIG. 9 illustrates a medium protection method according to an exemplaryembodiment of the present invention.

FIG. 10 illustrates a concept diagram of a downlink frame beingtransmitted based on DL MU OFDMA according to an exemplary embodiment ofthe present invention.

FIG. 11 illustrates a concept diagram of a downlink frame beingtransmitted based on DL MU OFDMA according to an exemplary embodiment ofthe present invention.

FIG. 12 illustrates a concept diagram of a downlink frame beingtransmitted based on DL MU OFDMA according to an exemplary embodiment ofthe present invention.

FIG. 13 illustrates a concept diagram showing a downlink frametransmission method of an AP according to an exemplary embodiment of thepresent invention.

FIG. 14 illustrates a concept diagram showing a PPDU format fortransmitting a frame according to an exemplary embodiment of the presentinvention.

FIG. 15 illustrates a block diagram showing a wireless communicationsystem in which the disclosure of this specification is implemented.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 illustrates a concept diagram illustrating the structure of awireless local area network (WLAN).

An upper portion of FIG. 1 shows the structure of the IEEE(institute ofelectrical and electronic engineers) 802.11 infrastructure network.

Referring to the upper portion of FIG. 1, the WLAN system may includeone or more basic service sets (BSSs, 100 and 105). The BSS 100 or 105is a set of an AP such as AP (access point) 125 and an STA such as STA1(station) 100-1 that may successfully sync with each other tocommunicate with each other and is not the concept to indicate aparticular area. The BSS 105 may include one AP 130 and one or more STAs105-1 and 105-2 connectable to the AP 130.

The infrastructure BSS may include at least one STA, APs 125 and 130providing a distribution service, and a distribution system (DS) 110connecting multiple APs.

The distribution system 110 may implement an extended service set (ESS)140 by connecting a number of BSSs 100 and 105. The ESS 140 may be usedas a term to denote one network configured of one or more APs 125 and230 connected via the distribution system 110. The APs included in oneESS 140 may have the same SSID (service set identification).

The portal 120 may function as a bridge that performs connection of theWLAN network (IEEE 802.11) with other network (for example, 802.X).

In the infrastructure network as shown in the upper portion of FIG. 1, anetwork between the APs 125 and 130 and a network between the APs 125and 130 and the STAs 100-1, 105-1, and 105-2 may be implemented.However, without the APs 125 and 130, a network may be establishedbetween the STAs to perform communication. The network that isestablished between the STAs without the APs 125 and 130 to performcommunication is defined as an ad-hoc network or an independent BSS(basic service set).

A lower portion of FIG. 1 is a concept view illustrating an independentBSS.

Referring to the lower portion of FIG. 1, the independent BSS (IBSS) isa BSS operating in ad-hoc mode. The IBSS does not include an AP, so thatit lacks a centralized management entity. In other words, in the IBSS,the STAs 150-1, 150-2, 150-3, 155-1, and 155-2 are managed in adistributed manner. In the IBSS, all of the STAs 150-1, 150-2, 150-3,155-1, and 155-2 may be mobile STAs, and access to the distributionsystem is not allowed so that the IBSS forms a self-contained network.

The STA is some functional medium that includes a medium access control(MAC) following the IEEE (Institute of Electrical and ElectronicsEngineers) 802.11 standards and that includes a physical layer interfacefor radio media, and the term “STA” may, in its definition, include bothan AP and a non-AP STA (station).

The STA may be referred to by various terms such as mobile terminal,wireless device, wireless transmit/receive unit (WTRU), user equipment(UE), mobile station (MS), mobile subscriber unit, or simply referred toas a user.

FIG. 2 illustrates a concept diagram showing a method for using a RTSframe and a CTS frame for resolving a hidden node issue and an exposednode issue.

Referring to FIG. 2, in order to solve the hidden node issue and theexposed node issue, a short signaling frame such as a request to send(RTS) frame, a clear to send (CTS) frame, or the like may be used.Neighboring STAs may know whether to transmit or receive data betweentwo STAs on the basis of the RTS frame and the CTS frame.

FIG. 2(A) illustrates a method of transmitting an RTS frame 203 and aCTS frame 205 to solve a hidden node issue.

It may be assumed a case where both of an STA A 200 and an STA C 220intend to transmit a data frame to an STA B 210. The STA A 200 maytransmit the RTS frame 203 to the STA B 210 before transmission of thedata frame, and the STA B 210 may transmit the CTS frame 205 to the STAA200. The STA C 220 may overhear the CTS frame 205, and may know frametransmission from the STA A 200 to the STA B 210 through a medium. TheSTA C 220 may configure a network allocation vector (NAV) until the endof data frame transmission from the STA A 200 to the STA B 210. Aninter-frame collision caused by a hidden node can be avoided by usingthis method.

FIG. 2(B) illustrates a method of transmitting an RTS frame 233 and aCTS frame 235 to solve an exposed mode issue.

An STA C 250 may determine whether a collision occurs when a frame istransmitted to another STA D 260 on the basis of monitoring of the RTSframe 233 and CTS frame 235 of an STAA 230 and an STAB 240.

The STA B 240 may transmit the RTS frame 233 to the STAA 230, and theSTAA 230 may transmit the CTS frame 235 to the STA B 240. The STA C 250overhears only the RTS frame 233 transmitted by the STA B 240, and failsto overhear the CTS frame 235 transmitted by the STAA 230. Therefore,the STA C 250 may know that the STAA 230 is located out of a carriersensing range of the STA C 250. Accordingly, the STA C 250 may transmitdata to the STA D 260.

An RTS frame format and a CTS frame format are disclosed in the 8.3.1.2RTS frame format and 8.3.1.3 CTS frame format of IEEEP802.11-REVmcTM/D2.0, October 2013.

FIG. 3 illustrates a concept diagram showing a CTS-to-Self Mechanism.

Referring to FIG. 3, a comparison between a case of sensing a medium byusing an exchange method between a RTS frame and a CTS frame ((A) ofFIG. 3) and a case of sensing a medium by using a CTS-to-Self frame ((B)of FIG. 3).

A CTS-to-self protection mechanism is defined in the IEEE 802.11gspecification. The CTS-to-self protection mechanism may be used insteadof the medium sensing mechanism, which uses a RTS frame and a CTS frame.In case of using the CTS-to-self protection mechanism, the overhead ofthe medium may be reduced as compared to when using the medium sensingmechanism, which uses RTS/CTS frames.

Referring to (A) of FIG. 3, the method for exchanging the RTS frame andthe CTS frame before the transmitting end transmits the data frame maybe performed as described below.

In (A) of FIG. 3, a case when STAA 300 attempts to send a data frame toSTA B 305 or STA C 310 will be assumed.

1) STAA 300 transmits a RTS frame 320.

2) The RTS frame 320 is received by STA B 305 and STA C 310, which isexist in a carrier sensing range.

3) STAB 305 and STA C 310 transmit CTS frames 325 and 330.

4) The transmitted CTS frames 325 and 330 are transmitted to STAA 300,STA B 305, STA C 310, and STA D 315.

In case of STA D 315, since STA D 315 exists outside of the carriersensing range of STAA 300, STA D 315 was unable to receive the RTS frame320 transmitted by STAA 300 (i.e., STA D 315 is a hidden node of STAA300). Nevertheless, by receiving the CTS frame 330 transmitted by STA C310, it will be apparent that STAA 300 has occupied the medium in orderto transmit data. STA D may configure a NAV and may not access themedium.

5) STAA 300 transmits a data frame to STA C 310.

Referring to (b) of FIG. 3, a CTS-to-self frame based medium sensingmethod, which is performed before the transmitting end transmits thedata frame, may be performed as described above. In (B) of FIG. 3, acase when STAA 300 attempts to send a data frame to STA C 360 will beassumed.

1) STAA 350 transmits a CTS-to-self frame 370, which exists in thecarrier sensing range, to STAB 355 and STA C 360.

2) After receiving the CTS-to-self frame 370, STA B 355 and STA C 360delay the transmitted of another data frame in order to receive the dataframe that is being transmitted from the STAA 350.

In case of using the above-described method, STA D 365, which existsoutside of the coverage region of STAA 350, in unable to receive theCTS-to-self frame 370 from STAA 350. Therefore, STA D 365 cannot knowwhether or not the data frame has been transmitted by STAA 350.

In this case, when STA D 365 transmits a data frame to STAA 350 or STA C360, a collision may occur between data frames. More specifically, themethod of using a CTS-to-self frame 370 cannot resolve the hidden nodeissue. Therefore, method of using a CTS-to-self frame 370 may be appliedonly to the case when the STAs are capable of sensing the data frametransmission occurring between one another, and, in other cases, themedium may be sensed by using the method of exchanging RTS/CTS frames.

An AP(access point) that operates in a wireless local area network(WLAN) system may transmit different sets of data to each of themultiple STAs(stations) through the same (or overlapping) time resource.If the transmission from the AP to the STA is referred to as a downlinktransmission, such transmission of the AP may be expressed by using theterm DL MU transmission (or downlink multi-user transmission).

In the legacy wireless system, the AP was capable of performing DL MUtransmission based on MU MIMO(multiple input multiple output), and suchtransmission may be expressed by using the term DL MU MIMO transmission.In the exemplary embodiment of the present invention, the AP may performDL MU transmission based on OFDMA(orthogonal frequency divisionmultiplexing access), and such transmission may be expressed by usingthe term DL MU OFDMA transmission. In case of using the DL MU OFDMAtransmission, the AP may transmit a downlink frame to each of themultiple STAs through each of the multiple frequency resources withinthe overlapping time resource.

Each of the PPDU, frame, and data being transmitted via downlinktransmission may be respectively expressed by using the terms downlinkPPDU, downlink frame, and downlink data. The PPDU may correspond to adata unit including a PPDU header and a PDSU(physical layer service dataunit) (or MPDU(MAC protocol data unit)). The PPDU header may include aPHY header and a PHY preamble, and the PDSU (or MPDU) may include aframe or may indicate a frame.

Conversely, the transmission from the STA to the AP may be referred toas an uplink transmission. The data transmission of multiple STAs to theAP within the same (or overlapping) time resource may be expressed byusing the term UL MU transmission (or uplink multi-user transmission).Unlike in the legacy wireless LAN system, the UL MU transmission may besupported in the wireless LAN system according to the exemplaryembodiment of the present invention. Each of the PPDU, frame, and databeing transmitted via uplink transmission may be respectively expressedby using the terms uplink PPDU, uplink frame, and uplink data. Theuplink transmission performed by each of the multiple STAs may berespectively performed within different frequency domains (subbands) orwithin different spatial domains.

In case the uplink transmission performed by each of the multiple STAsis respectively performed within different frequency domains (ordifferent subbands), different frequency resources respective to each ofthe multiple STAs may be allocated as the uplink transmission resourcebased on OFDMA. Each of the multiple STAs may respectively transmit anuplink frame to the AP through each of the different frequency resourcesallocated thereto. Such transmission method using different frequencyresources may also be expressed by using the term UL MU OFDMAtransmission method.

In case the uplink transmission performed by each of the multiple STAsis respectively performed within different spatial domains, differentspace time streams (or spatial streams) may be allocated with respect toeach of the multiple STAs. Each of the multiple STAs may respectivelytransmit an uplink frame to the AP through different space time streams.Such transmission method using different space time streams may also beexpressed by using the term UL MU MIMO transmission method.

In the next generation wireless LAN, demands for high throughput andenhanced QoE(quality of experience) are increasing. In case of adoptinga new frame (or PPDU) format for the next generation wireless LANsystem, the design of a new system should be realized without anyperformance impact from legacy STAs, which support only the legacywireless LAN system. Additionally, the next generation wireless LANsystem is also required to be designed not to be influenced by anyperformance impact caused by the presence of a legacy STA.

As described above, the DL MU OFDMA transmission was not supported inthe legacy (or conventional) wireless LAN system. In the legacy wirelessLAN system, a wider bandwidth based on multi-channels was allocated forthe communication between one STA and one AP. The multi-channel maycorrespond to a bandwidth exceeding 20 MHz and including a primarychannel and a secondary channel.

In the legacy wireless LAN system, restrictions (or limitations) existedin the operation of a wider bandwidth due to a primary channel rule.According to the primary channel rule, the STA may perform communicationthrough the multi-channel including the secondary channel only in a casewhen the secondary channel is idle. More specifically, in case thesecondary channel, which is adjacent to the primary channel, is beingused in an OBSS(overlapped BSS) (i.e., in case the secondary channel isbusy), the bandwidth cannot be expanded to a multi-channel, and the STAmay perform communication only through the primary channel STA. In casethe primary channel rule is being applied, it may be difficult tooperate a wider bandwidth in an environment having more than a fewOBSSs.

Accordingly, instead of a single STA, in case multiple STAs performcommunication through a multi-channel simultaneously (or within anoverlapping time resource) based on DL MU OFDMA, the usage efficiency ofthe frequency resource may be increased.

In case using the DL MU OFDMA transmission, it may be importance todecide OFDMA subband granularity. In case the DL MU is performed basedon the OFDMA transmission, the OFDMA subband granularity may correspondto a subband (or frequency resource) unit being allocated for thecommunication of one STA.

In case a subband having a size smaller than the basic 20 MHz unit,which was used in the legacy wireless LAN system, is being used for theDL MU OFDMA communication, a problem may occur in the co-existence ofthe legacy STA, which is operated based on the legacy 20 MHz frequencyband, and the STA, which supports DL MU OFDMA communication, and whichis operated based on a frequency band less than 20 MHz. For example, aproblem may occur with respect to medium protection (or channelprotection) based on a RTS frame and a CTS frame.

Considering backward compatibility, in the next generation wireless LANsystem, which operates based on DL MU OFDMA, for a successful receptionof a RTS frame or a CTS frame of the legacy STA and for a NAVconfiguration, the RTS frame (or PPDU) or the CTS frame (CTS PPDU) maymaintain the conventional (or legacy) frame format or (PPDU format).

The exemplary embodiment of the present invention discloses a mediumprotection method that is based on the RTS frame and CTS frame in anenvironment where a legacy STA and an STA supporting the next generationwireless LAN system co-exist. The legacy STA may correspond to an STAthat can be operated within a multi-channel of a 20 MHz bandwidth unit,which is decided based on the primary channel rule. The STA supportingthe next generation wireless LAN system may also be operated within asubband less than 20 MHz (or a subband of 20 MHz or more) based on DL MUOFDMA or UL MU OFDMA. Hereinafter, in the exemplary embodiment of thepresent invention, unless it is separately expressed as a legacy STA,the STA may refer to an STA supporting the next generation wireless LANsystem.

Additionally, hereinafter, in the exemplary embodiment of the presentinvention, a case when the OFDMA subband granularity is equal to 5 MHzwill be assumed. More specifically, one STA may perform communicationwith the AP based on a minimum subband of 5 MHz. Nevertheless,communication based on DL MU OFDMA and/or UL MU OFDMA, which is based onan OFDMA subband granularity of 20 MHz or less or 20 MHz or more,instead of 5 MHz, may also be performed.

Additionally, hereinafter, although a case when a downlink frame (ordownlink PPDU) is respectively transmitted to each of the multiple STAsby the AP based on DL MU OFDMA is assumed in the exemplary embodiment ofthe present invention, the medium protection method based on the RTSframe/CTS frame according to the exemplary embodiment of the presentinvention may also be used in a case when a downlink frame (or downlinkPPDU) is respectively transmitted to each of the multiple STAs by the APthrough different space time streams based on DL MU MIMO.

FIG. 4 illustrates a concept diagram showing a medium protection methodwhen performing OFDMA communication based communication in a wirelessLAN according to an exemplary embodiment of the present invention.

FIG. 4 discloses a method of sequentially transmitting CTS frames 410,420, 430, and 440 in response to a RTS frame 400.

Referring to FIG. 4, the AP may transmit a RTS frame 400 to multipleSTAs configuring an OFDMA packet. In other words, the AP may transmit aRTS frame 400 to each of the multiple STAs, which are to respectivelytransmit multiple downlink frames, based on the DL MU OFDMAtransmission. Hereinafter, each of the multiple STAs configuring anOFDMA packet, each of the multiple STAs that are to be respectivelyreceive each of the downlink frames, which are transmitted by the APbased on DL MU OFDMA transmission, may be expressed by using the term DLMU target STA.

The RTS frame 400 may include identification information respective toeach of the multiple DL MU target STAs (e.g., MAC address respective toeach of the multiple DL MU target STAs or an ID respective to each ofthe multiple DL MU target STAs (AID(association identifier),PAID(partial association identifier)).

A RA field of the RTS frame 400 may have a format for includingidentification information respective to multiple DL MU target STAsinstead of the identification information of one DL MU target STA. Incase the RTS frame 400 maintains its conventional format, the legacy STAmay decode only the duration field, which is located before the RA fieldof the RTS frame 400, so as to configure the NAV. Therefore, even incase the format respective to the RA field of the RTS frame 400 ischanged, there may be no impact (or influence) on the legacy STA.

Each the multiple DL MU target STAs may sequentially transmit CTS frames410, 420, 430, and 440 in response to the RTS frame 400. Thetransmission order of the CTS frames 410, 420, 430, and 440 of themultiple DL MU target STAs may be decided based on diverse methods. Forexample, the transmission order of the CTS frames 410, 420, 430, and 440of the multiple DL MU target STAs may be decided based on an index (orposition) of the allocated subband. For example, in case the OFDMAsubband granularity is equal to 5 MHz, a frequency bandwidth of 20 MHzmay include 4 subbands, and a subband index respective to each of the 4subbands may correspond to Subband 1, Subband 2, Subband 3, and Subband4.

In order to establish communication between STA1 to STA4 and the AP, acase when Subband 1 is allocated to STA1, when Subband 2 is allocated toSTA2, when Subband 3 is allocated to STA3, and when Subband 4 isallocated to STA4 may be assumed. The subbands that are allocated toeach of the STAs for the DL MU transmission may be decided based on theRTS frame 400 or may be decided based on an exchange procedure ofanother frame prior to the transmission of the RTS frame/CTS frame. Inthis case, in accordance with the order of the subband index, each ofSTA1, STA2, STA3, and STA4 may sequentially transmit CTS frames 410,420, 430, and 440. STA1 receives the RTS frame 400, and, after a SIFS(short interframe space), STA1 may transmit CTS Frame 1 410 to the AP.Thereafter, STA2, STA3, and STA4 may sequentially transmit CTS Frame 2420, CTS Frame 3 430, and CTS Frame 4 440. More specifically, STA2monitors the transmission of CTS Frame 1 410 performed by STA1, and,then, after a SIFS after the transmission of CTS Frame 1 410, STA2 maytransmit CTS Frame 2 420. Alternatively, considering the subband indexthat is allocated after the reception of the RTS frame 400, STA2 decidesthe transmission timing of CTS Frame 2 420, and, then, STA2 may transmitCTS Frame 2 420 at the decided transmission timing of CTS Frame 2 420.

According to another exemplary embodiment of the present invention, thetransmission order of the CTS frames 410, 420, 430, and 440 of themultiple DL MU target STAs may be decided based on an order ofidentification information respective to each of the multiple DL MUtarget STAs included in the RA field of the RTS frame 400. For example,a plurality of bits configuring the RA field of the RTS frame 400 maysequentially indicate STA1, STA2, STA3, and STA4 as the multiple DL MUtarget STAs. In this case, considering the order indicated by the RAfield, each of STA1, STA2, STA3, and STA4 may transmit CTS Frame 1 410,CTS Frame 2 420, CTS Frame 3 430, and CTS Frame 4 440 to the AP by theorder of STA1, STA2, STA3, and STA4.

The CTS frames 410, 420, 430, and 440 may not include a TA(transmitteraddress) field including identification information of the transmittingend (or each of the multiple DL MU target STAs transmitting the CTSframes 410, 420, 430, and 440). In this case, as described above, the APmay acquire information respective to the DL MU STAs, which havetransmitted the CTS frames 410, 420, 430, and 440 based on thepre-decided transmission order of the CTS frames 410, 420, 430, and 440or the reception timing of the CTS frames 410, 420, 430, and 440.

The RTS frame 400 and the CTS frames 410, 420, 430, and 440 may betransmitted through the entire bandwidth for the transmission of the RTSframe 400 and the CTS frames 410, 420, 430, and 440. More specifically,instead of being transmitted based on OFDMA, the RTS frame 400 and theCTS frames 410, 420, 430, and 440 may be transmitted through an entirebandwidth that is allocated based on one PPDU. The format of such PPDUmay be expressed by using the term non-duplicate PPDU format.Alternatively, the RTS frame and the CTS frames may also be transmittedthrough multiple channels based on a duplicate PPDU format. Theduplicate PPDU format may replicate a PPDU format being transmittedthrough a neighboring channel (or primary channel) (20 MHz) and may thenbe transmitted through a bandwidth exceeding 20 MHz (e.g., 40 MHz, 80MHz, 160 MHz, 80 MHz+80 MHz, and so on). In case the duplicate format isused, one PPDU including the data that are replicated through each ofthe multiple channels (replication target channel and replicatechannel).

The RTS PPDU and CTS PPDU of the non-duplicate PPDU format or theduplicate PPDU format may be transmitted to at least one STA (or AP)through at least one space time stream.

According to another exemplary embodiment of the present invention, theRTS frame 400 may also be transmitted based on a DL MU transmissionmethod. More specifically, in case of the RTS frame 400, different RTSframes 400 may be respectively transmitted to multiple STAs throughdifferent space time streams based on the DL MU MIMO transmission.Alternatively, different RTS frames 400 may be respectively transmittedto multiple STAs through different frequency resources (or subbands,channels) based on DL MU OFDMA. In this case, the RTS frames 400 beingtransmitted through different space time streams or different frequencyresources may not include the same information. More specifically, theAP may respectively transmit each of the multiple RTS frames 400 to eachof the STAs based on the DL MU transmission. For example, a RTS frame400 that is being transmitted through a specific space time stream or aspecific frequency resource based on the DL MU transmission may onlyindicate a specific STA that is to receive the RTS frame 400 through aspecific frequency resource.

The AP may respectively receive CTS frames 410, 420, 430, and 440 frommultiple DL MU target STAs and may transmit downlink frames (or downlinkdata frames or downlink management frames) based on the DL MU OFDMAtransmission.

In FIG. 4, a case when the RTS frame 400 is transmitted to multiple DLMU target STAs, when each of the multiple DL MU target STAs that hasreceived the RTS frame 400 sequentially transmits the CTS frames 410,420, 430, and 440 to the AP, and when the CTS frames 410, 420, 430, and440 that are transmitted by each of the multiple DL MU target STAs aresuccessfully decoded in the AP, is assumed.

The AP may transmit each of the multiple downlink frames (downlinkPPDUs) to each of the multiple DL MU target STAs within the overlappingtime resource through the subbands allocated to each of the multiple DLMU target STAs.

For example, the AP may transmit Downlink Frame 1 (Downlink PPDU 1) 450to STA1 through Subband 1, the AP may transmit Downlink Frame 2(Downlink PPDU 2) 460 to STA2 through Subband 2, the AP may transmitDownlink Frame 3 (Downlink PPDU 3) 470 to STA3 through Subband 3, andthe AP may transmit Downlink Frame 4 (Downlink PPDU 4) 480 to STA4through Subband 4.

FIG. 5 illustrates a concept diagram of a RTS frame according to anexemplary embodiment of the present invention.

Referring to FIG. 5, the RTS frame may include a frame control field500, a duration field 510, a RA(receiver address) field 520, aTA(transmitter address) field 530, and a FCS(frame check sequence) field540.

The frame control field 500 may include information for indicating theRTS frame.

The duration field 510 may include duration information for thetransmission of a CTS frame, an uplink transmission indication frame,uplink frames respective to each of the multiple STAs, and an ACK frame.As described in the exemplary embodiment of the present invention, incase multiple CTS frames are sequentially received, the duration field510 may include duration information that has considered the receptionof multiple CTS frames.

The RA field 520 may include information indicating the DL MU target STAthat is to transmit the CTS frame. For example, the RA field 520 maycorrespond to a field of 48 bits (6 octets). In case a maximum of 4 STAssupport the UL MU transmission, among the 48 bits, 12 bits may beallocated in order to identify one STA. As described above, the DL MUtarget STA may transmit the CTS frame while considering the order of theidentifiers of the multiple STA, which are included in the received RAfield.

The TA field 530 may include an address of the AP transmitting the RTSframe.

The FCS field 540 may include information for verifying the validity ofa frame.

FIG. 6 and FIG. 7 disclose collisions that may occur between a BSSsupporting the DL MU OFDMA transmission and a neighboring BSS that doesnot support the DL MU OFDMA transmission.

FIG. 6 illustrates a concept diagram showing a collision between a BSSsupporting DL MU OFDMA transmission and a BSS not supporting DL MU OFDMAtransmission.

Referring to FIG. 6, in BSS1 supporting the DL MU OFDMA transmission,AP1 615 may transmit a RTS frame to multiple DL MU target STAs (STA1610, STA2 620, STA3 630, and STA4 640), and, in response to the RTSframe, AP1 615 may receive a CTS frame from each of the multiple DL MUtarget STAs (STA1 610, STA2 620, STA3 630, and STA4 640).

In BSS2 that does not support the DL MU OFDMA transmission, STA5 650 mayreceive a RTS frame, which is transmitted by AP1 615, and may thenconfigure a NAV. In BSS2, STA6 660 may only receive the CTS frame, whichwas transmitted by STA4 640. If STA6 acquires the authority to access amedium through a back-off procedure prior to the reception of the CTSframe of STA6 660 (or the transmission of the CTS frame of STA4), STA6660 may transmit an uplink frame to AP2 625 without configuring a NAV.In this case, a collision may occur between the uplink frame transmittedby STA6 660 and the downlink frame transmitted by AP1 615 in thereceiving end of STA4 640.

FIG. 7 illustrates a concept diagram showing a collision between a BSSsupporting DL MU OFDMA transmission and a BSS not supporting DL MU OFDMAtransmission.

Referring to FIG. 7, in BSS1 supporting the DL MU OFDMA transmission,AP1 715 may transmit a RTS frame to multiple DL MU target STAs (STA1710, STA2 720, STA3 730, and STA4 740), and, in response to the RTSframe, AP1 715 may receive a CTS frame from each of the multiple DL MUtarget STAs (STA1 710, STA2 720, STA3 730, and STA4 740).

In BSS2 that does not support the DL MU OFDMA transmission, STA5 750 mayreceive a RTS frame, which is transmitted by AP1 715, and may thenconfigure a NAV. AP2 725 may be incapable of receiving the RTS frametransmitted from AP1 715, and AP2 725 may receive the CTS frametransmitted by STA4 740. In case AP2 725 acquires the authority toaccess a medium through a back-off procedure before receiving the CTSframe (or before the transmission of the CTS frame by STA4 740), AP2 725may transmit a downlink frame to STA6 720 without configuring a NAV. Inthis case, a collision may occur between the downlink frame transmittedby AP2 725 and the downlink frame transmitted by AP1 715 in thereceiving end of STA4 740.

Hereinafter, the exemplary embodiment of the present invention disclosesa method for preventing the problem of collision between frames shown inFIG. 6 and FIG. 7.

Hereinafter, in the exemplary embodiment of the present invention, BSS 1may indicate a BSS that supports the DL MU OFDMA, and BSS2 may indicatea BSS that does not support the DL MU OFDMA and that is adjacent to BSS1or that overlaps with BSS1.

FIG. 8 illustrates a medium protection method according to an exemplaryembodiment of the present invention.

FIG. 8 discloses a method for deciding whether or not the DL MU targetSTA, which is included in BSS1, should transmit a CTS frame in responseto the RTS frame based on the frame that is being transmitted by BSS2.

As described above in the procedure of FIG. 5, in BSS1, AP1 815 maytransmit the RTS frame to multiple DL MU target STAs. According to theexemplary embodiment of the present invention, among the multiple DL MUtarget STAs that have received the RTS frame, a DL MU target STA, whichhas detected the occupation of the medium by the AP or STA included inBSS2, may not transmit a CTS frame.

For example, after the reception of the RTS frame that is transmitted byAP1 815, which is included in BSS1, STA4 840 may receive the RTS framethat is transmitted by STA5 850. In this case, STA4 840 may not transmita CTS frame in response to the RTS frame, which is transmitted by AP1815. Alternatively, STA4 840 may transmit a CTS frame. However, the CTSframe may include additional information indicating that the receptionof a downlink frame cannot be performed (or the reception of thedownlink frame is not available).

In case AP1 815 fails to receive a CTS frame from STA4 840, it will beapparent that a downlink frame cannot be transmitted to STA4 840 throughany one of the subbands included in the entire band (e.g., 20 MHz) thathas transmitted the RTS frame. More specifically, in BSS2, since thelegacy STA (STA5 850) and AP2 825 performs communication through aminimum band of 20 MHz, a collision between the frames may occurregardless of the subband, which is included within 20 MHz, that is usedby AP1 815 to transmit a downlink frame to STA4. Therefore, AP1 cannottransmit a downlink frame to STA4.

FIG. 8 discloses a method for not transmitting a CTS frame, in a casewhen a DL MU STA (STA4 840), which has received the RTS frametransmitted by AP1 815, detects a RTS frame being transmitted by STA5850, which is included in BSS2, before transmitting the CTS frame.

However, in addition to the RTS frame transmitted by STA5 850, the STA4840 may detect a CTS frame of STA5 850 being transmitted as a responseto the RTS frame, which is transmitted by AP2 825, and an ACK frame ofSTA5 850 being transmitted as a response to the downlink frame, which istransmitted by AP2 825, and may not transmit the CTS frame to AP1 815.

Among the multiple DL MU target STAs, in case a DL MU target STA thatdoes not transmit a CTS frame exists, the AP may transmit a downlinkframe to the remaining DL MU target STAs excluding the DL MU target STAthat does not transmit a CTS frame based on DL MU OFDMA. In the caseshown in FIG. 8, in case the subband being allocated to STA4 840corresponds to Subband 4, AP1 815 may respectively transmit DownlinkFrame 1 to STA1 810, Downlink Frame 2 to STA2 820, and Downlink Frame 3to STA3 830 through the remaining subbands (Subbands 1, 2, and 3)excluding Subband 4 based on DL MU OFDMA.

FIG. 8 discloses a method according to which the DL MU target STAdetects a frame being transmitted by a STA included in BSS2 and does nottransmit a CTS frame. The DL MU target STA may also detect a frame beingtransmitted by an AP included in BSS2 and may not transmit a CTS frame.

More specifically, the AP may transmit a RTS(request to send) for mediumprotection to a first STA(station) set, and the AP may sequentiallyreceive a CTS(clear to send) from each of the multiple STAs included ina second STA set in response to the RTS frame.

The first STA set may include DL MU target STAs that are indicated basedon the RTS frame, which is transmitted by the AP. The RA field of theRTS frame may include identification information of the multiple STAsincluded in the first STA set. Among the multiple DL MU target STAs, thesecond STA set may include STAs that have transmitted a CTS frame to theAP. The remaining STAs that are included in the first STA set and yetnot included in the second STA set may correspond to DL MU target STAs,which have detected the occupation of the medium by another AP oranother STA, and which may configure a NAV and not transmit the CTSframe.

The AP may respectively transmit each of the multiple PPDUs to each ofthe multiple STAs being included in the second STA set through each ofthe multiple subbands for each of the multiple STAs within theoverlapping time resource. The AP may transmit data units includingdummy signals through subbands that are allocated to the remaining STAs,which are included in the first STA set yet not included in the secondSTA set within the overlapping time resource. Alternatively, the AP mayalso decide a transmission power respective to each of the multiplePPDUs while considering the size of the subbands being allocated to theremaining STAs, which are included in the first STA set yet not includedin the second STA set within the overlapping time resource. This will bedescribed later on in more detail.

FIG. 9 illustrates a medium protection method according to an exemplaryembodiment of the present invention.

FIG. 9 discloses a method for deciding whether or not the DL MU targetSTA, which is included in BSS1, should transmit a CTS frame in responseto the RTS frame based on the frame that is being transmitted by BSS2.

As described above in the procedure of FIG. 5, in BSS1, the AP maytransmit the RTS frame to multiple DL MU target STAs. Among the multipleDL MU target STAs that have received the RTS frame, a DL MU target STA,which has detected the occupation of the medium by the AP or STAincluded in BSS2, may not transmit a CTS frame.

For example, after the reception of the RTS frame that is transmitted byAP1 915, which is included in BSS1, STA4 940 may receive the RTS framethat is transmitted by AP2 925. In this case, STA4 940 may not transmita CTS frame in response to the RTS frame, which is transmitted by AP1915. Alternatively, STA4 940 may transmit a CTS frame. However, the CTSframe may include additional information indicating that the receptionof a downlink frame cannot be performed (or the reception of thedownlink frame is not available). In case AP1 915 fails to receive a CTSframe from STA4 940, it will be apparent that a downlink frame cannot betransmitted to STA4 940.

FIG. 9 discloses a method for not transmitting a CTS frame, in a casewhen a DL MU STA, which corresponds to STA4 940, that has received theRTS frame transmitted by AP1 915, detects a RTS frame being transmittedby AP2 925, which is included in BSS2, before transmitting the CTSframe.

However, in addition to the RTS frame transmitted by AP2 925, STA4 940may detect a CTS frame of AP2 925 being transmitted as a response to theRTS frame, which is transmitted by STA5 950, and an ACK frame of AP2 925being transmitted as a response to the downlink frame, which istransmitted by STA5 950, and may not transmit the CTS frame.

Among the multiple DL MU target STAs, in case a DL MU target STA thatdoes not transmit a CTS frame exists, the AP may transmit a downlinkframe to the remaining DL MU target STAs excluding the DL MU target STAthat does not transmit a CTS frame based on DL MU OFDMA.

FIG. 10 illustrates a concept diagram of a downlink frame beingtransmitted based on DL MU OFDMA according to an exemplary embodiment ofthe present invention.

FIG. 10 discloses a method according to which the AP transmit a downlinkframe based on DL MU OFDMA, in a case when a DL MU target STA that hasnot transmitted a CTS frame exists among the multiple DL MU target STAs.

Referring to FIG. 10, the AP may transmit a RTS frame 1000 to DL MUtarget STAs (STA1, STA2, STA3, and STA4). The AP may receive CTS Frame 11010, CTS Frame 2 1020, and CTS Frame 3 1030, which are respectivetransmitted by STA1, STA2, and STA3, and the AP may not receive CTSFrame 4 from STA4 1040.

In this case, the AP may transmit a downlink frame to each of STA 1,STA2, and STA3 with the exception for STA4. For example, the AP maytransmit Downlink Frame 1 (Downlink PPDU 1) 1040 to STA1 through Subband1, may transmit Downlink Frame 2 (Downlink PPDU 2) 1050 to STA2 throughSubband 2, and may transmit Downlink Frame 3 (Downlink PPDU 3) 1060 toSTA3 through Subband 3, based on the DL MU OFDMA transmission with theoverlapping time resource. The AP may not transmit Downlink Frame 4(Downlink PPDU 4) through Subband 4.

FIG. 11 illustrates a concept diagram of a downlink frame beingtransmitted based on DL MU OFDMA according to an exemplary embodiment ofthe present invention.

FIG. 11 discloses a method according to which the AP transmit a downlinkframe based on DL MU OFDMA, in a case when a DL MU target STA that hasnot transmitted a CTS frame exists among the multiple DL MU target STAs.

Referring to FIG. 11, the AP may transmit a RTS frame 1100 to DL MUtarget STAs (STA1, STA2, STA3, and STA4). The AP may receive CTS Frame 11110, CTS Frame 2 1120, and CTS Frame 3 1130, which are respectivetransmitted by STA1, STA2, and STA3, and the AP may not receive CTSFrame 4 from STA4.

In case a downlink PPDU is not transmitted through Subband 4, as shownin FIG. 10, the detection levels of the STAs detecting downlink frames,which are being transmitted based on DL MU OFDMA, may be interpreteddifferently. For example, the STA may differently determine whether themedium is busy or idle in accordance with the number of multiplesubframes being transmitted based on DL MU OFDMA. In case the STA hassensed a power level having a predetermined size within the medium basedon a CCA(clear channel assessment), the STA may determine that themedium is busy. In this case, the size of the power, which is sensedwithin the medium in accordance with the number of multiple subframes,may vary, and the interpretation of the state of the medium respectiveto the STAs may vary accordingly.

In order to resolve such imbalance in the transmission power, thetransmission power of the downlink frame (or downlink PPDU) may beadjusted in accordance with the number of multiple subframes beingtransmitted based on DL MU OFDMA. For example, the transmission powerfor transmitting Downlink Frame 4 (Downlink PPDU 4) through Subband 4may be used for the transmission of another one of Downlink Frame 1 toDownlink Frame 3 (Downlink PPDU 1 1140 to Downlink PPDU 3 1160). Morespecifically, the transmission power for each of Downlink PPDU 1 1140 toDownlink PPDU 3 1160 may be increased while considering the transmissionpower of Downlink PPDU 4.

Alternatively, the AP may also transmit a data unit 1170 including adummy signal through Subband 4. More specifically, the AP may determinewhether or not to include a dummy signal based on whether or not the CTSframe has been received from the DL MU target STA.

STA4, which detects a frame that is being transmitted by the STA or APincluded in BSS2, and which does not transmit a CTS frame to the AP, mayalready be in a state of configuring a NAV based on a frame, which istransmitted by the STA or AP included in BSS2. Therefore, thetransmission of the dummy signal may not cause any influence (or impact)of the performance of STA4.

FIG. 12 illustrates a concept diagram of a downlink frame beingtransmitted based on DL MU OFDMA according to an exemplary embodiment ofthe present invention.

In FIG. 12, a CTS frame may be transmitted based on a MBSFN(multicastbroadcast single frequency network) method.

Referring to FIG. 12, the AP may transmit a RTS frame 1200 to DL MUtarget STAs (STA1, STA2, STA3, and STA4). A DL MU target STA maytransmit a CTS frame 1210 to the AP by using the MBSFN method. Morespecifically, the CTS frame 1210 that is being transmitted by the DL MUtarget STA may include the same data and may be transmitted from anoverlapping time resource and an overlapping frequency resource (orwithin the same (or overlapping) subband). In FIG. 11, a case when STA4does not transmit CTS Frame 4 will be assumed.

The RA field of the CTS frame 1210 may include identificationinformation (e.g., MAC address) of a transmitting end (e.g., AP) of theRTS frame 1200 being included in the TA field of the RTS frame 1200.

Each of STA1 to STA3 may receive the RTS frame 1200 and may transmit aCTS frame 1210 including the same data in each field after a SIFS. TheCTS frame 1210 may not include a field including information on theaddress of a transmitting end (e.g., TA(transmitter address) field),which has transmitted the CTS frame 1210. Therefore, the AP may beincapable of knowing by which DL MU target STA, among the multiple DL MUtarget STAs that have received the RTS frame 1200, the CTS frame 1210has been transmitted.

The AP may transmit a downlink frame to all of the multiple DL MU targetSTAs, which are targeted by the RTS frame 1200, without considering theDL MU target STA that has not transmitted the CTS frame 1210. Referringto FIG. 12, the AP may also transmit a downlink frame to STA4, which hasnot transmitted the CTS frame 1210, through Subband 4. The CTS frame1210 may also be transmitted for the NAV configuration of another STAexcluding the DL MU target STAs.

More specifically, the AP may transmit a RTS(request to send) frame formedium protection to a first STA(station) set. The RA(receiver address)field of the RTS frame being transmitted by the AP may includeidentification information respective to each of the multiple STAs beingincluded in the first STA set. As a response to the RTS frame, the APmay receive a CTS frame from each of the multiple STAs being included inthe second STA set within an overlapping time resource and anoverlapping frequency resource, and, herein, the second STA set may beincluded in the first STA set. The first STA set may include DL MUtarget STAs from which the AP of the RTS frame intends to receive theCTS frame (or to which the AP intends to transmit a downlink frame). Thesecond STA set may include STAs transmitting the CTS frame as a responseto the RTS frame.

After receiving a CTS frame from the STA included in the second STA set,the AP may transmit each of the multiple PPDUs to each of the multipleSTAs included in the first STA set through each of the multiple subbandsfor each of the multiple STAs being included in the first STA set withinan overlapping time resource.

More specifically, the AP may transmit downlink frames (downlink PPDUs)1250, 1260, 1270, and 1280 to all of the multiple DL MU target STAsbased on the DL MU OFDMA transmission without considering whether or notthe reception of the CTS frame 1210, which is transmitted by all of themultiple DL MU target STAs that are targeted by the RTS frame 1200, issuccessful.

Referring to FIG. 8 and FIG. 9, STA4, which detects the frame beingtransmitted by the STA (e.g., STA5 in FIG. 8) or the AP (e.g., AP2 inFIG. 9) that is included in BSS2 and does not transmit a CTS frame tothe AP, may already be in a state of configuring a NAV based on a frame,which is transmitted by the STA or AP included in BSS2. Therefore, thetransmission of the Downlink Frame 4 may not cause any influence (orimpact) of the performance of STA4.

Additionally, the frame being transmitted by the STA (e.g., STA5 in FIG.8) or the AP (e.g., AP2 in FIG. 9) that is included in BSS2 does notcause any interference on the DM MU target STA receiving a downlinkframe and being included in the BSS.

The method disclosed in FIG. 12 may effectively support the NAVconfiguration of a legacy STA. Therefore, this method may be moreeffective in a BSS or ESS(extended service set), wherein a large numberof legacy STAs exist.

FIG. 13 illustrates a concept diagram showing a downlink frametransmission method of an AP according to an exemplary embodiment of thepresent invention.

In case multiple downlink PPDUs are transmitted based on DL MU OFDMAwithout any medium protection based on the RTS frame and the CTS frame,FIG. 13 discloses a medium protection method.

Referring to FIG. 13, when the AP transmits multiple downlink PPDUsbased on DL MU OFDMA, a NAV configuration respect to another STA(including legacy STAs) instead of a hidden node (or hidden position)may be supported.

The PPDU carrying (or containing (or including) a frame may include alegacy part and a non-legacy part. The legacy part may include L-STF1310, L-LTF 1320, and L-SIG 1330. The non-legacy part 1340 may include aPPDU header (e.g., HE-STF, HE-LTF, and HE-SIG) for a non-legacy wirelessLAN system that supports DL MU OFDMA and UL MU OFDMA (or UL MU MIMO),and a MPDU(MAC protocol data unit) (or PSDU(physical layer service dataunit)). The non-legacy part 1340 may also be expressed by using the termHEW part (or HE part). The non-legacy part 1340 may include HE-SIG A,HE-STF, HE-LTF, and HE-SIG B, which will be described later on in moredetail.

The legacy part 1310, 1320, and 1330 may be transmitted through a 20 MHzband and may be decoded by a legacy STA. The non-legacy part 1340 may betransmitted to multiple STAs based on DL MU OFDMA based on a subbandgranularity that is smaller than 20 MHz (e.g., 5 MHz).

The legacy STA may receive the legacy part 1310, 1320, and 1330, mayconfigure a NAV based on a legacy field (e.g., length information, MCSinformation, and so on, included in the legacy field), and may notperform decoding on the non-legacy part 1340.

The STA (or non-legacy STA) may convert the legacy part 1310, 1320, and1330, which is being transmitted through a 20 MHz band, based on 64FFT,and may perform decoding on the converted part, and the STA may alsoperform decoding on the remaining non-legacy part 1340, thereby beingcapable of receiving data.

The legacy STA may not successfully perform decoding starting from thenon-legacy part 1340. Accordingly, the legacy STA may know that thereceived PPDU correspond to a PPDU that is not supported (that the PPDUdoes not correspond to its packet). Alternatively, a most temporallypreceding block within the non-legacy part 1340 (or a most temporallypreceding OFDM symbol or most temporally preceding field within thenon-legacy part 1340) may be transmitted through 20 MHz so that theconstellation rotation rule can be applied.

In case of applying the constellation rotation rule, a PPDU format (or aversion of the wireless LAN system that has transmitted the PPDU) may bedetected based on the constellation that was used for generating amodulation symbol that is being transmitted through the OFDM symbol. Forexample, a symbol that is being transmitted within a last OFDMA symbol(or last field) of the legacy part may be generated based on a firstconstellation, and a first OFDMS symbol (or first field) of thenon-legacy part may be generated based on a second constellation.

In order to apply the constellation rotation rule, the first OFDM symbolor first field of the non-legacy part 1340 may be transmitted from a 20MHz band. In other words, in order to allow the constellation rotationrule to be applied, the first OFDM symbol for first field of thenon-legacy part 1340 may correspond to a common non-legacy block (or HEblock) of 20 MHz.

The first field of the non-legacy part may correspond to HE-STF orHE-LTF for differentiating the bandwidth or may correspond to HE-SIGincluding identification information of multiple STAs that are toreceive downlink frames.

FIG. 14 illustrates a concept diagram showing a PPDU format fortransmitting a frame according to an exemplary embodiment of the presentinvention.

FIG. 14 discloses a PPDU format according to an exemplary embodiment ofthe present invention. The PPDU format disclosed in FIG. 14 may carry orinclude a downlink frame, which is transmitted based on the DL MU OFDMAtransmission. Alternatively, the RTS frame and the CTS frame accordingto the exemplary embodiment of the present invention may also betransmitted through the PPDU format, which is disclosed in FIG. 14.

Referring to the upper part of FIG. 14, the PHY header of the downlinkPPDU may include a L-STF(legacy-short training field), aL-LTF(legacy-long training field), a L-SIG(legacy-signal), a HE-SIGA(high efficiency-signal A), a HE-STF(high efficiency-short trainingfield), a HE-LTF(high efficiency-long training field), and a HE-SIGB(high efficiency-signal-B). The PHY header may be divided into a legacypart, which consists of a part up to L-SIG, and a HE(high efficiency)part (HE part), which consists of after the L-SIG.

The L-STF 1400 may include a short training OFDM symbol (short trainingorthogonal frequency division multiplexing symbol). The L-STF 1400 maybe used for frame detection, AGC(automatic gain control), diversitydetection, and coarse frequency/time synchronization.

The L-LTF 1410 may include a long training OFDM symbol (long trainingorthogonal frequency division multiplexing symbol). The L-LTF 1410 maybe used for fine frequency/time synchronization and channel prediction.

The L-SIG 1420 may be used for transmitting control information. TheL-SIG 1420 may include information on data transmission rate, datalength, and so on. As described above, the legacy STA may configure aNAV based on the information included in L-SIG.

The HE-SIG A 1430 may include information for indicating the STA that isintended to receive the PPDU. For example, the HE-SIG A 1430 may includean identifier of a specific STA that is to receive the PPDU, informationfor indicating the group of the STA. Additionally, in case the PPDU istransmitted based on OFDMA or MIMO, the HE-SIG A 1430 may also includeresource allocation information respective to the STA.

Additionally, the HE-SIG A 1430 may also include color bits informationfor BSS identification information, bandwidth information, tail bit, CRCbit, MCS(modulation and coding scheme) information respective to theHE-SIG B 1460, information on the number of symbols for the HE-SIG B1460, and CP(cyclic prefix) (or GI(guard interval)) length information.

The HE-STF 1440 may be used for enhancing automatic gain controlestimation in a MIMO(multiple input multiple output) environment or anOFDMA environment.

The HE-LTF 1450 may be used for estimating a channel in a MIMOenvironment or an OFDMA environment.

The HE-SIG B 1460 may include information on a length MCS of aPSDU(Physical layer service data unit) respective to each STA and tailbit, and so on. Additionally, the HE-SIG B 1460 may also includeinformation on an STA that is to receive the PPDU, OFDMA based resourceallocation information (or MU-MIMO information). In case the OFDMA basedresource allocation information (or MU-MIMO related information) isincluded in the HE-SIG B 1460, the resource allocation information maynot be included in the HE-SIG A 1430.

The size of the IFFT being applied to the HE-STF 1440 and the fieldafter the HE-STF 1440 may be different from the size of the IFFT beingapplied to the field before the HE-STF 1440. For example, the size ofthe IFFT being applied to the HE-STF 1440 and the field after the HE-STF1440 may be four time larger than the size of the IFFT being applied tothe field before the HE-STF 1440. The STA may receive the HE-SIG A 1430and may receive indications on the reception of a downlink PPDU based onthe HE-SIG A 1430. In this case, the STA may perform decoding based on aFFT size, which is changed starting from the HE-STF 1440 and the fieldafter the HE-STF 1440. Conversely, in case the STA fails to receiveindication on the reception of a downlink PPDU based on the HE-SIG A1430, the STA may stop the decoding process and may configure aNAV(network allocation vector). The CP(cyclic prefix) of the HE-STF 1440may have a size that is larger than the CP(cyclic prefix) of anotherfield, and, during such CP interval, the STA may change the FFT size soas to perform decoding on the downlink PPDU.

The order of the field configuring the format of the PPDU, which isdisclosed on the upper part of FIG. 14, may vary. For example, as shownin the middle part of FIG. 14, the HE-SIG B 1415 of the HE part may belocated immediately after the HE-SIG A 1405. The STA may performdecoding on the HE-SIG A 1405 and up to the HE-SIG B 1415, so as toreceive the required control information, and, then, the STA mayconfigure the NAV. Similarly, the size of the IFFT being applied to theHE-STF 1425 and the field after the HE-STF 1425 may be different fromthe size of the IFFT being applied to the field before the HE-STF 1425.

The STA may receive the HE-SIG A 1405 and the HE-SIG B 1415. In case thereception of the PPDU is indicated based on the HE-SIG A 1405, the STAmay change the FFT size starting from the HE-STF 1425 and may performdecoding on the PPDU. Conversely, in case the STA receives the HE-SIG Abut fails to receive indication on the reception of the downlink PPDUbased on the HE-SIG A 1405, the STA may configure a NAV(networkallocation vector).

Referring to a lower part of FIG. 14, a PPDU format for DL MUtransmission is disclosed. According to the exemplary embodiment of thepresent invention, the AP may transmit a downlink frame or a downlinkPPDU to multiple STAs by using the PPDU format for DL MU transmission.Each of the multiple downlink PPDUs may be respectively transmitted toeach of the multiple STAs through different transmission resources(frequency resources or space time streams). Within the PPDU, the fieldbefore the HE-SIG B 1445 may be transmitted in a duplicated format fromtransmission resources each being different from one another. The HE-SIGB 1445 may be transmitted in an encoded format within the entiretransmission resource. The field after the HE-SIG B 1445 may includeindividual information for each of the multiple STAs receiving the PPDU.

In case the fields included in the PPDU are respectively transmittedthrough each of the transmission resources, a CRC respective to eachfield may be included in the PPDU. Conversely, in case a specific fieldincluded in the PPDU is encoded within the entire transmission resourceand then transmitted, a CRC respective to each field may not be includedin the PPDU. Accordingly, an overhead respective to the CRC may bereduced.

Similarly, in the PPDU format or the DL MU transmission, the HE-STF 1455and the field after the HE-STF 1455 may be encoded based on an IFFT sizethat is different from the field before the HE-STF 1455. Therefore, incase the STA receives the HE-SIG A 1435 and the HE-SIG B 1445, and incase the STA receives an indication to receive the PPDU based on theHE-SIG A 1435, the STA may change the FFT size starting from the HE-STF1455 and may then perform decoding on the PPDU.

FIG. 15 illustrates a block diagram showing a wireless communicationsystem in which the disclosure of this specification is implemented.

Referring to FIG. 15, as an STA that can realize the above-describedexemplary embodiment, the wireless device 1500 may correspond to an AP1500 or a non-AP STA (non-AP station) 1550.

The AP 1500 includes a processor 1510, a memory 1520, and a RF unit(radio frequency unit) 1530.

The RF unit 1530 is connected to the processor 1510, thereby beingcapable of transmitting and/or receiving radio signals.

The processor 1510 implements the functions, processes, and/or methodsproposed in the present invention. For example, the processor 1510 maybe realized to perform the operations of the wireless device accordingto the above-described exemplary embodiments of the present invention.The processor may perform the operations of the wireless device, whichare disclosed in the exemplary embodiments of FIG. 1 to FIG. 14.

For example, the processor 1510 may be configured to transmit aRTS(request to send) frame for medium protection to a first STA(station)group and to sequentially receive CTS(clear to send) frames from each ofmultiple STAs being included in a second STA set as a response to theRTS frame. And, the processor 1510 may also be configured torespectively transmit each of multiple PPDUs (physical layer protocoldata unit) to each of the multiple STAs through each of multiplesubbands for each of the multiple STAs within an overlapping timeresource. The second STA set may be included in the first STA set.

The STA 1550 includes a processor 1560, a memory 1570, and a RF unit(radio frequency unit) 1580.

The RF unit 1580 is connected to the processor 1560, thereby beingcapable of transmitting and/or receiving radio signals.

The processor 1560 implements the functions, processes, and/or methodsproposed in the present invention. For example, the processor 1560 maybe realized to perform the operations of the wireless device accordingto the above-described exemplary embodiments of the present invention.The processor may perform the operations of the wireless device, whichare disclosed in the exemplary embodiments of FIG. 1 to FIG. 14.

For example, the processor 1560 may be implemented to receive a RTSframe from the AP and to transmit the CTS frame at a transmissiontiming, which is decided based on a position within the frequencyresource of a subband for the STA.

The processor 1510 and 1560 may include an ASIC(application-specificintegrated circuit), another chip set, a logical circuit, a dataprocessing device, and/or a converter converting a baseband signal and aradio signal to and from one another. The memory 1520 and 1570 mayinclude a ROM(read-only memory), a RAM(random access memory), a flashmemory, a memory card, a storage medium, and/or another storage device.The RF unit 1530 and 1580 may include one or more antennas transmittingand/or receiving radio signals.

When the exemplary embodiment is implemented as software, theabove-described method may be implemented as a module (process,function, and so on) performing the above-described functions. Themodule may be stored in the memory 1520 and 1570 and may be executed bythe processor 1510 and 1560. The memory 1520 and 1570 may be locatedinside or outside of the processor 1510 and 1560 and may be connected tothe processor 1510 and 1560 through a diversity of well-known means.

What is claimed is:
 1. A method for transmitting a frame in a wirelessLAN, comprising: transmitting, by an AP(access point), a RTS(request tosend) frame for medium protection to a first STA(station) group;sequentially receiving, by the AP, CTS(clear to send) frames from eachof multiple STAs being included in a second STA set as a response to theRTS frame, wherein the second STA set is included in the first STA set;and respectively transmitting, by the AP, each of multiple PPDUs(physical layer protocol data unit) to each of the multiple STAs througheach of multiple subbands for each of the multiple STAs within anoverlapping time resource.
 2. The method of claim 1, wherein aRA(receiver address) field of the RTS frame comprises identificationinformation respective to each of the multiple STAs included in thefirst STA set.
 3. The method of claim 1, wherein the AP transmits a dataunit including a dummy signal through a subband allocated to remainingSTAs being included in the first STA set yet not included in the secondSTA set.
 4. The method of claim 1, wherein the AP decides a transmissionpower for each of the multiple PPDUs based on a size of a subbandallocated to remaining STAs being included in the first STA set yet notincluded in the second STA set.
 5. The method of claim 1, wherein eachof the multiple STAs transmits the CTS frame at a transmission timingbeing decided based on a position of each of multiple subbands for eachof the multiple STAs within a frequency resource.
 6. An AP(access point)transmitting a frame in a wireless LAN, comprising: a RF(radiofrequency) unit being configured to transmit or receive radio signals;and a processor being operatively connected to the RF unit, wherein theprocessor is configured: to transmit a RTS(request to send) frame formedium protection to a first STA(station) group, to sequentially receiveCTS(clear to send) frames from each of multiple STAs being included in asecond STA set as a response to the RTS frame, and to respectivelytransmit each of multiple PPDUs (physical layer protocol data unit) toeach of the multiple STAs through each of multiple subbands for each ofthe multiple STAs within an overlapping time resource, and wherein thesecond STA set is included in the first STA set.
 7. The AP of claim 6,wherein a RA(receiver address) field of the RTS frame comprisesidentification information respective to each of the multiple STAsincluded in the first STA set.
 8. The AP of claim 6, wherein the APtransmits a data unit including a dummy signal through a subbandallocated to remaining STAs being included in the first STA set yet notincluded in the second STA set.
 9. The AP of claim 6, wherein the APdecides a transmission power for each of the multiple PPDUs based on asize of a subband allocated to remaining STAs being included in thefirst STA set yet not included in the second STA set.
 10. The AP ofclaim 6, wherein each of the multiple STAs transmits the CTS frame at atransmission timing being decided based on a position of each ofmultiple subbands for each of the multiple STAs within a frequencyresource.
 11. A method for transmitting a frame in a wireless LAN,comprising: transmitting, by an AP(access point), a RTS(request to send)frame for medium protection to a first STA(station) group; receiving, bythe AP, CTS(clear to send) frames from each of multiple STAs beingincluded in a second STA set as a response to the RTS frame within anoverlapping time resource and an overlapping frequency resource, whereinthe second STA set is included in the first STA set; and respectivelytransmitting, by the AP, each of multiple PPDUs (physical layer protocoldata unit) to each of the multiple STAs included in the first STA setthrough each of multiple subbands for each of the multiple STAs includedin the first STA set within an overlapping time resource, wherein aRA(receiver address) field of the RTS frame comprises identificationinformation respective to each of the multiple STAs included in thefirst STA set.
 12. An AP(access point) transmitting a frame in awireless LAN, comprising: a RF(radio frequency) unit being configured totransmit or receive radio signals; and a processor being operativelyconnected to the RF unit, wherein the processor is configured: totransmit a RTS(request to send) frame for medium protection to a firstSTA(station) group, to receive CTS(clear to send) frames from each ofmultiple STAs being included in a second STA set as a response to theRTS frame within an overlapping time resource and an overlappingfrequency resource, and to respectively transmit each of multiple PPDUs(physical layer protocol data unit) to each of the multiple STAsincluded in the first STA set through each of multiple subbands for eachof the multiple STAs included in the first STA set within an overlappingtime resource, wherein a RA(receiver address) field of the RTS framecomprises identification information respective to each of the multipleSTAs included in the first STA set, and wherein the second STA set isincluded in the first STA set.